How can I code a program that will encode a text written only in capital letters according to the text below?

I created an char array of 256 elements, to account for all ASCII characters. I stored corresponding value of the character in the array. For example: A-->11, alpha_num['A'] = 11. Similarily, I assigned all the remaining characters.

#include <stdio.h>
#include <string.h>

int main()
{
    int i = 1, j = 1;
    int k = 0;
    char alpha_num[256];
    char string[20];
    printf("Please enter an input\n");
    scanf("%s", string);

    for (char c = 'A'; c <= 'Z'; c++)
    {
        if (j <= 6 && i <= 7)
        {
            alpha_num[c] = i * 10 + j;
            j++;
        }
        if (j > 6)
        {
            j = 1;
            i++;
        }

        if (i > 7)
            i = 1;
    }

    i = 5;
    j = 2;
    for (char c = '1'; c <= '9'; c++)
    {
        if (j <= 6 && i <= 7)
        {
            alpha_num[c] = i * 10 + j;
            j++;
        }
        if (j > 6)
        {
            j = 1;
            i++;
        }

        if (i > 7)
            i = 1;
    }

    alpha_num['?'] = 65;
    alpha_num['!'] = 66;
    alpha_num['-'] = 71;
    alpha_num['+'] = 72;
    alpha_num['*'] = 73;

    for (int i = 0; i < strlen(string); i++)
    {
        printf("%d", alpha_num[string[i]]);
    }

    return 0;
}

The output is :

Please enter an input
HELLO1234
221526263352535455

I went for short and simple:

(IDEOne Link)

#include <stdio.h>
#include <string.h>

void encode(char* input, char* output, char* matrix)
{
    while(*input)
    {
        size_t idx = strchr(matrix, *input++) - matrix;
        *output++ = '1' + idx / 6;
        *output++ = '1' + idx % 6;
    }
    *output = 0;
}

int main(void) {
    char matrix[]="ABCDEFGHIJKLMNOPRSTUVYZWX123456789?!-+* ";
    char input[50];
    
    printf("please enter an input");
    fgets(input, 50, stdin);
    *strchr(input, '\n') = '\0';
    
    size_t len = strlen(input);
    char output[2*len +1];
    
    encode(input, output, matrix);
    printf("\nEncoded String: %s\n", output);
    return 0;
}

 #include <stdio.h>

struct CodeList {
   int code;   
   int key;
};

int main()
{
   struct CodeList  list[6*7];
   int i = 1;
   int y =1;
   int x =1;

   for (char c = 'A'; c <='Z'; c++){
        if (x  > 6) {
           x =1;
           y++;

       }

        if (y  > 7) {
           y =1;
       }
       list[i].key = c;  
       list[i].code = (y * 10) + x;
       i++;
       x++;
  
   }
   
   //search
   char s = 'D';
   
   for (i =0; i< 6*7; i++){
       if(s == list[i].key){
         printf("code %i", list[i].code);// 14

       }
   }
}

By the way, you ignored the letter Q.

You can iterate over string characters and check if the current char is found in b. If it's found you can just print the row+1 and the column+1:

#include <stdio.h>
#include <string.h>

char b[7][6]={'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','R','S','T','U'
  ,'V','Y','Z','W','X','1','2','3','4','5','6','7','8','9','?','!','-','+','*'};

void findRowCol(char c,int *row,int *col){// change row,col to the founded index, if not found to -1
     *row=-1;
     *col=-1;
     for(int i=0;i<7;i++){
         for(int j=0;j<6;j++){
            if(c==b[i][j]){
              *row=i;
              *col=j;
          }
       }
    }
}

int main()
{
    char string[20] = "HELLO1234";

    for(int i=0;i<strlen(string);i++){
       int row,col;
       findRowCol(string[i],&row,&col);
       if(row!=-1) printf("%d%d",row+1,col+1);   
    }
}